Thrombosis accounts for about 40% of the deaths in the United States. Current treatments for thrombotic disorders involve the use of anticoagulant drugs (e.g., heparin, coumadin) that have non-specific mechanisms of action. These drugs can cause bleeding thus limiting their use. Anticoagulants that block early steps in the coagulation cascade, and are specific for either the intrinsic or extrinsic pathways (see FIG. 1), could have superior efficacy and safety profiles because thrombotic events are suppressed without inducing bleeding episodes.